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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

Posted on 22 September 2010 by alan_marshall

Following the failure to reach a strong agreement at the Copenhagen conference, climate skeptics have had a good run in the Australian media, continuing their campaigns of disinformation. In such an atmosphere it is vital that we articulate the basic science of climate change, the principles of physics and chemistry which the skeptics ignore.

The purpose of this article is to clearly explain, in everyday language, the two key principles which together determine the rate at which temperatures rise. The first principle is the greenhouse effect of carbon dioxide and other gases. The second principle is the thermal inertia of the oceans, sometimes referred to as climate lag. Few people have any feel for the numbers involved with the latter, so I will deal with it in more depth.

The Greenhouse Effect

The greenhouse effect takes its name from the glass greenhouse, which farmers have used for centuries, trapping heat to grow tomatoes and other plants that could not otherwise be grown in the colder regions of the world. Like glass greenhouses, greenhouse gases allow sunlight to pass through unhindered, but trap heat radiation on its way out. The molecular structure of CO2 is such that it is “tuned” to the wavelengths of infrared (heat) radiation emitted by the Earth’s surface back into space, in particular to the 15 micrometer band. The molecules resonate, their vibrations absorbing the energy of the infra-red radiation. It is vibrating molecules that give us the sensation of heat, and it is by this mechanism that heat energy is trapped by the atmosphere and re-radiated to the surface. The extent to which temperatures will rise due to a given change in the concentration of greenhouse gases is known as the “climate sensitivity,” and you may find it useful to search for this term when doing your own research.

Most principles of physics are beyond question because both cause and effect are well understood. A relationship between cause and effect is proved by repeatable experiments. This is the essence of the scientific method, and the source of knowledge on which we have built our technological civilization. We do not question Newton’s laws of motion because we can demonstrate them in the laboratory. We no longer question that light and infrared radiation are electromagnetic waves because we can measure their wavelengths and other properties in the laboratory. Likewise, there should be no dissent that CO2 absorbs infrared radiation, because that too has been demonstrated in the laboratory. In fact, it was first measured 150 years ago by John Tyndall [i] using a spectrophotometer. In line with the scientific method, his results have been confirmed and more precisely quantified by Herzberg in 1953, Burch in 1962 and 1970, and others since then.

Given that the radiative properties of CO2 have been proven in the laboratory, you would expect them to be same in the atmosphere, given that they are dependent on CO2’s unchanging molecular structure. You would think that the onus would be on the climate skeptics to demonstrate that CO2 behaves differently in the atmosphere than it does in the laboratory. Of course they have not done so. In fact, since 1970 satellites have measured infrared spectra emitted by the Earth and confirmed not only that CO2 traps heat, but that it has trapped more heat as concentrations of CO2 have risen.

The above graph clearly shows that at the major wavelength for absorption by CO2, and also at wavelength for absorption by methane, that less infrared was escaping in to space in 1996 compared to 1970.

After 150 years of scientific investigation, the impact of CO2 on the climate is well understood. Anyone who tells you different is selling snakeoil.

The Thermal Inertia of the Oceans

If we accept that greenhouse gases are warming the planet, the next concept that needs to be grasped is that it takes time, and we have not yet seen the full rise in temperature that will occur as a result of the CO2 we have already emitted. The Earth’s average surface temperature has already risen by 0.8 degrees C since 1900. The concentration of CO2 in the atmosphere is increasing at the rate of 2 ppm per year. Scientists tell us that even if CO2 was stabilized at its current level of 390 ppm, there is at least another 0.6 degrees “in the pipeline”. If findings from a recent study of Antarctic ice cores is confirmed, the last figure will prove to be conservative [ii]. The delayed response is known as climate lag.

The reason the planet takes several decades to respond to increased CO2 is the thermal inertia of the oceans. Consider a saucepan of water placed on a gas stove. Although the flame has a temperature measured in hundreds of degrees C, the water takes a few minutes to reach boiling point. This simple analogy explains climate lag. The mass of the oceans is around 500 times that of the atmosphere. The time that it takes to warm up is measured in decades. Because of the difficulty in quantifying the rate at which the warm upper layers of the ocean mix with the cooler deeper waters, there is significant variation in estimates of climate lag. A paper by James Hansen and others [iii] estimates the time required for 60% of global warming to take place in response to increased emissions to be in the range of 25 to 50 years. The mid-point of this is 37.5 which I have rounded to 40 years.

In recent times, climate skeptics have been peddling a lot of nonsense about average temperatures actually cooling over the last decade. There was a brief dip around the year 2000 following the extreme El Nino event of 1998, but with greenhouse emissions causing a planetary energy imbalance of 0.85 watts per square metre [iv], there is inevitably a continual rising trend in global temperatures. It should then be no surprise to anyone that the 12 month period June 2009 to May 2010 was the hottest on record [v].

The graph below from Australia’s CSIRO [vi] shows a clear rising trend in temperatures as well as a rising trend in sea-level.

Implications of the 40 Year Delay

The estimate of 40 years for climate lag, the time between the cause (increased greenhouse gas emissions) and the effect (increased temperatures), has profound negative consequences for humanity. However, if governments can find the will to act, there are positive consequences as well.

With 40 years between cause and effect, it means that average temperatures of the last decade are a result of what we were thoughtlessly putting into the air in the 1960’s. It also means that the true impact of our emissions over the last decade will not be felt until the 2040’s. This thought should send a chill down your spine!

Conservative elements in both politics and the media have been playing up uncertainties in some of the more difficult to model effects of climate change, while ignoring the solid scientific understanding of the cause. If past governments had troubled themselves to understand the cause, and acted in a timely way, climate change would have been contained with minimal disruption. By refusing to acknowledge the cause, and demanding to see the effects before action is taken, past governments have brought on the current crisis. By the time they see those effects, it will too late to deal with the cause.

The positive consequence of climate lag is the opportunity for remedial action before the ocean warms to its full extent. We need to not only work towards reducing our carbon emissions to near zero by 2050, but well before then to begin removing excess CO2 from the atmosphere on an industrial scale. Biochar is one promising technology that can have an impact here. Synthetic trees, with carbon capture and storage, is another. If an international agreement can be forged to provide a framework for not only limiting new emissions, but sequestering old emissions, then the full horror of the climate crisis may yet be averted.

Spreading the Word

The clock is ticking. All of us who understand clearly the science of climate change, and its implications for humanity, should do what we can to inform the public debate. I wrote the original version of this article in February 2010 to help inform the Parliament of Australia. The letter was sent to 40 MPs and senators, and has received positive feedback from both members of the three largest parties. To find out more about this information campaign, and for extensive coverage of the science of climate change and its technological, economic and political solutions, please visit my web site at www.climatechangeanswers.org.

References

i Gulf Times, “A Last Chance to Avert Disaster”, available at www.gulf-times.com/site/topics/article.asp? cu_no=2&item_no=330396&version=1&template_id=46&parent_id=26

"With 40 years between cause and effect, it means that average temperatures of the last decade are a result of what we were thoughtlessly putting into the air in the 1960’s."

I think it was this statement in the post that gave me a case of cognitive dissonance, especially when juxtaposed with:

"The Earth’s average surface temperature has already risen by 0.8 degrees C since 1900. ... there is at least another 0.6 degrees “in the pipeline”."

So there isn't a full T+40 delay to see the onset of warming; its T+40 to see the full effect. Would it therefore be correct to say that the warming we see now is the sum total of the onset of warming from recent emissions, the tail end of warmings from older emissions and some fraction of everything in between? So that the 'lag' mentioned, which I took to be a delay time, is more like a 'storage time'? A good analogy would then be a circuit with large inductance; it takes considerable time for the current to diminish to 0 after the driving voltage is shut off.

00

Moderator Response: Author:
Your understanding of climate lag is now almost correct. Your observation that "warming we see now is the sum total of the onset of warming from recent emissions, the tail end of warmings from older emissions and some fraction of everything in between" is a helpful way to put it. The only thing you need to realise is that technically, the full effect is never reached, only approached asymptotically. The climate model in which I place most faith projects the temperature rise for 1500 years, at which point it regards equilibrium as having been reached. We need to be practical. The 40 years in the lead article is the time from the onset of warming for the temperature rise to reach 63.2% or (1 – 1/e) of the full effect. See my comment # 54.

muoncounter,
your analogy should work. Add that you lower (or rise) the voltage in steps or continuosly and you need to integrate the response over time. The climate response to a varying forcing is basically similar.

In the lead article I used the paper by James Hansen and others (ref iii) as a credible estimate for climate lag. His paper states:

"Evidence from Earth's history and climate models suggests that climate sensitivity is 0.75° ± 0.25°C per W/m2, implying that 25 to 50 years are needed for Earth's surface temperature to reach 60% of its equilibrium response."

My article qualifies this estimate with the comments about the "difficulty in quantifying the rate at which the warm upper layers of the ocean mix with the cooler deeper waters". When I wrote the piece, I was also aware of a paper by B. Lin and others * that uses a different assumption about ocean heat transport. It states:

"The estimated time constant of the climate is large (70 - 120 years) mainly owing to the deep ocean heat transport …"

CBDunkerson’s comment #9 is useful here. The figure he provides of 0.027 W/m2 for the deep basins is a great deal less than the 0.85 W/m2 at the top of the atmosphere. Therefore I reserve judgment on Lin’s estimate of the time constant, and have chosen to use that provided by Hansen as a starting point for debate.

Now the time constant is the time it takes for the system's step response to reach 1 – 1/e = 63.2% of its final equilibrium value. If we take the mid-point of Hansen’s range for 60% warming, 37.5 years, and convert it to a 63.2% warming, by my calculations the resulting time constant is 40.9 years.

There is not really any joy for skeptics here. All the papers make clear that at this point in time, we are still talking about estimates. The point of the article is that climate lag is real, it is measured in decades, and it has provided cover for skeptics who dispute the science of global warming. Climate lag means our situation is more perilous than most of the public perceive, and decision-makers need to be made aware. In 5 years or so, I expect we will have more deep ocean data, and be able to estimate the time constant with more certainty. In the meantime, I think 40 years is an appropriate number with which to engage the public.

* Atmospheric Chemistry and Physics Discussions, Estimations of climate sensitivity based on top-of-atmosphere radiation imbalance, available at http://www.atmos-chem-phys.org/10/1923/2010/acp-10-1923-2010.pdf

Nerndt at 14 and others,
The oceans temperature is controlled by its interaction with the atmosphere. AGW causes the atmosphere to warm. When the atmosphere warms, it warms the ocean under it. Because the ocean is so much larger than the atmosphere, it takes a long time for the ocean to warm to equilibrium. The time lag described in this article is largely due to the long time it takes for the ocean to be warmed by the hot atmosphere. The ocean cools the atmosphere as it warms up. When the ocean finally reaches equilibrium, the atmosphere is hotter than it was at the start because the ocean no longer cools it. Since we live in the atmosphere, we are most concerned with the temperature rise in the atmosphere. The rise in atmospheric temperatures is slowed by the thermal mass of the ocean.

Thus the atmosphere warms immediately when the forcing on it increases. As the ocean (slowly) warms, the atmosphere warms even more, even if the forcing no longer rises.

Unfortunately, the IPCC estimates of climate sensitivity of 2-4.5C per doubling are for short term warming, not equilibrium warming. The equilibrium warming, in say 1000 years, is higher than the short term warming.

The article provides good coverage of how CO2 causes heating, but unfortunately, it does nto actually address the fallacies 'skeptics' usually rely on for disbelieving that there is a greenhouse effect. Less time on unconstested physics and more rebuttal of the current memes would be a good idea.

Energy consumption was erratic between 1910-1950, with a gradual upward trend. Temperature swings between 1950-1990 were also erratic with a general upward trend. From 1950 on, energy consumption has been increasing at a nearly uniform rate until 1990. The rate of increase started declining after 1990. From 1990 on, global temperatures have been increasing at a nearly uniform rate.

Only time will tell what happens in the next 40 years, considering that energy consumption is still increasing. And we must not forget that the methane gas factor may enter the picture after global temperatures have risen above a certain level. When or if CH4 reaches 6 ppm, with its additional CO2 byproduct, global temperatures could rise even faster than predicted by energy consumption alone.

This article is extremely poorly worded. The title says there is a 40 year delay between cause and effect, which makes it sound like the warming effect of co2 doesn't *start* until 40 years after co2 starts rising. Obviously that's wrong. But throughout the article this impression is given. Eg:

"The reason the planet takes several decades to respond to increased CO2 is the thermal inertia of the oceans."

The planet doesn't respond *at all* to rising co2 until several decades after? Of course it does.

And:
"With 40 years between cause and effect, it means that average temperatures of the last decade are a result of what we were thoughtlessly putting into the air in the 1960’s"

And the 1970s and the 1980s and the 1990s. Not just the 1960s.

Obviously what you mean is that there is a 40 year delay between cause and *maximum* effect.

I only say this because I feel strongly that the way the article is worded, only people who understand this already are going to get it. People who are new to this or are not but are easily confused (*cough* Steve Goddard *cough*), will get (or have already got) the wrong idea.

Perseus asked: "Could this same 40 year lag in increased tempeatures be used as an 'explanation' by a sceptic for the Solar influence which peaked way back in the last century?"

Response: "No, the way climate time lag works is when the planet is in energy imbalance (eg - more energy coming in than going out), the planet steadily accumulates heat and warms. As it warms, it radiates more heat out to space until eventually, the energy out equals the energy in and the planet is back in equilibrium. So the way climate time lag works is the planet gradually warms over decades and the energy imbalance gradually shrinks.

That's not what we've seen over the last half century. After solar activity peaked in the mid-20th century, the planet's energy imbalance - rather than shrink - has actually increased as CO2 levels have increased."

If you look at solar activity, to say it peaked in the mid-20th century you must be referring to sunspot activity. Whilst this is true in absolute terms, the average has continued to rise over the past 50 years, and would suggest the energy imbalance continues, as you have found. Average monthly sunspot activity from 2000-2010 is the highest seen since records began in the 1700s, and there was a rising trend since 1950, only starting to level off in the last few years since the Sun's activity has started to stall.

TSI is the measurement of solar activity, rising and falling with the 11-year solar (sunspot) cycle. For the 30 years or so of accurate measurements, TSI and sunspot activity are in good agreement. As such, sunspot activity is used a surrogate marker, or proxy, for TSI for periods predating the instrumental record.

TSI has been static (actually, fitting a trend line to TSI levels show it declining slightly) while global temps go up. As you can see here:

Question: The surface of the oceans is on average warmer than the near-surface air temperature. How can atmospheric heat warm the oceans?

(My guess is that circulation patterns from diurnal and latitudinal changes sea heat exchange from air to oceans under certain conditions, but I still wonder how the oceans are heating when, generally, the air is warmer than the water)

barry, water may be a poor conductor of heat, but there is always some conduction.

I'm sure someone has shown that one can warm water by having warm air at its surface (while keeping the bulk of the water beneath the surface insulated from any other source of heat).

It may be slow and inefficient, but there is no reason why water cannot be warmed - to some extent - by a warmer gas at its surface. When we're talking about such a huge surface area, that amounts to a lot of heat over months and years. Remembering that there's also direct (even if inefficient) heating by radiation.

Once you add in convection by ocean currents, there's plenty of scope for heating to affect the whole of the ocean.

A skeptical friend points out that the average temperature of the near-surface atmosphere is cooler than the average temperature of the sea surface (link). His question is: how can the atmosphere warm the oceans if the surface of the oceans are, on average, warmer than the atmosphere?

I figure the answer is 'circulation' - and that much of the heat transfer from atmosphere to ocean would occur at night and in warm regions. It's not been easy verifying these details on the net. General overviews, like the post heading this thread, are quite common.

This is a bit late but I think this article needs serious re-editing. It correctly quotes a paper by Hansen, et al with "the time required for 60% of global warming to take place in response to increased emissions to be in the range of 25 to 50 years" but later goes on the say "With 40 years between cause and effect" and this is reflected in the title. I'm not referring to the rounding to 40 years but to the implication that any CO2 that lodges in the atmosphere won't warm the surface at all for 40 years (or whatever the actual period is). This is surely incorrect. The CO2 has well known heat trapping properties, which are not put into abeyance for some decades. The lag is not a lag at all, it is simply that 60% of the ultimate effect takes some decades to become manifest. But some smaller percentage becomes manifest in a smaller time. Indeed, the CO2 begins trapping heat immediately.

Hansen et al (2011) ("Earth's Energy Imbalance and Implications") includes a graphic representation of the climate response to a doubling of CO2 (though I can't see why a similar response curve wouldn't apply to any increase in CO2 forcing). It shows a very rapid response to the forcing, reaching 60% in about something less than half a century, as far as I can tell, but 40% effect is reached within a few years.

So there is no lag at all; it's simply that it takes perhaps 40 years or so for most of the ulitimate effect to be felt.

muoncounter got it about right in an earlier comment but this didn't result in any editing of the article.